Photocatalytic roofing granules, photocatalytic roofing products and process for preparing same

ABSTRACT

Photocatalytic roofing granules include a binder and inert mineral particles, with photocatalytic particles dispersed in the binder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to roofing granules and roofing productsincluding roofing granules.

2. Brief Description of the Prior Art

Asphalt shingles are conventionally used in the United States and Canadaas roofing and siding materials. Roofing granules are typicallydistributed over the upper or outer face of such shingles. The roofinggranules, in general formed from mineral materials, serve to provide theshingle with durability. They protect the asphalt from the effects ofthe solar radiation (in particular from the degradative effects ofultraviolet rays) and of the environment (wind, precipitation,pollution, and the like), and contribute to better reflection ofincident radiation. The granules moreover are typically colored,naturally or artificially by way of the application of pigments, to meetthe aesthetic requirements of the user.

However, it is not unusual to see unattractive green, brown or blackspots appearing on the surface of asphalt shingles of buildings locatedin temperate climates. These spots are due to micro-organisms, mainlyalgae of the Gloeocapsa genus which benefit from conditions favorable totheir growth found in temperate climates. These conditions include heat,moisture and nutrients. The essential biogenic salts may be provided bythe mineral granules themselves, but also may be supplied by organicmatter which settles on the shingles. The unattractiveness of thesespots, all the more noticeable when the color of the shingle is a lightone, is not the only disadvantage. In addition, the resulting darkeningof the surface causes an increase in the absorption of the solarradiation, which in turn reduces the effectiveness of the shingles asthermal insulation, and decreases their service life.

To address this problem, algae-contaminated shingles can be treated withsuitable biocides. However, the complete elimination of the algae isdifficult, and requires the treatment of the entire building, includingseemingly healthy surfaces. Even by using a powerful biocide such assodium hypochlorite, the prophylactic effect is not permanent, becausethe roof is subsequently scrubbed by weather-borne water. Moreover,certain green algae particularly resistant to biocides can re-colonizepreviously treated surfaces, thus requiring additional treatments, atregular intervals, to limit their reappearance.

Other methods known to prevent the appearance of the undesirable algaegrowth are based on the incorporation of algaecide in the shingle. Forexample, it has been suggested that granules include metal compounds inthe form of zinc oxide or sulfide (U.S. Pat. No. 3,507,676), or copperoxide (U.S. Pat. No. 5,356,664), or that a mixture copper oxide and zincoxide (U.S. Patent Publication 2002/0258835 and U.S. Patent Publication2002/0255548) can be incorporated in the asphalt.

It has also been suggested to disperse a granular or pulverulentmaterial containing an algaecide on the surface of the shingle(JP-A-2004162482). U.S. Pat. No. 6,245,381 suggests adding a biocide inthe form of salt or of chelate starting from Cu²⁺, Zn²⁺ and Sn²⁺ ionscomplexed with an organic binder anion in asphalt during the manufactureof the shingle.

Another approach has been to employ photocatalytic particles as biocidalagents. The photocatalytic effect has been employed to provideself-cleaning glass and other ceramic materials. For example, U.S. Pat.No. 6,037,289 discloses a substrate provided with photocatalytic anatasetitanium dioxide that is at least partially crystalline, and has a meansize of between 5 and 80 nm. The coating can include an inorganicbinder, such as an amorphous or partially crystalline oxide, or mixtureof oxides, such as oxides of silicon, titanium, tin, zirconium oraluminum, which can serve as a matrix for the photocatalytic titaniumoxide. Alternatively, a partly organic binder can be used, such as abinder based on epoxide-containing alkoxysilanes. Similarly, U.S. Pat.No. 6,465,088 discloses a substrate such as a glass or acrylate glazingmaterial covered with a photocatalytic coating including crystallizedparticles having photocatalytic properties and a mineral bindercomprising at least one oxide of a metal having photocatalyticproperties. U.S. Pat. Nos. 6,569,520 and 6,881,702 disclose aphotocatalytic composition and method for preventing algae growth onbuilding materials such as roofing granules. A plurality ofphotocatalytic particles, such as anatase titanium dioxide, is dispersedin a silicate binder to form an exterior coating for a substrate such asa roofing granule or concrete surface. At least a portion of some of thephotocatalytic particles is exposed on the surface of the coating.

In general all these approaches aim to provide biocide at the surface ofthe roofing granules, but also require significant deviations from theconventional techniques for producing such granules, such asformulating, applying and curing one or more interior coatings includingbiocidal materials, adding functional components such as variousbiocidal materials to the exterior color coating composition used toprovide color to the granules and the roofing shingles formed with suchgranules, and the like.

Functional materials are substances that confer special or desirableproperties when added to a composition, such as coating composition.Biocides are an example of one class of functional materials. Anothertype of functional material encountered in the roofing granule artenhances the solar reflectance of the roofing granules. Some materialsmay have multiple functional characteristics.

Colored granules have been modified using functional materials toprovide special functions to the granules and the shingles or membranesthat contain these granules. The most common feature is algae resistancewhich relies on the metal oxides, such as cuprous oxide, to serve as thealgaecides. Solar reflectance is another feature that has been added tothe roofing granules by incorporating solar reflective or solartransparent pigments. The major disadvantage of these types offunctionalized colored granules is the high cost—usually 10 to 20-foldmore expensive than the standard colored granules. The main reason is acombination of complicated manufacturing processes in order to achievethe desired colors and properties, plus the high costs of raw materials(algaecides and/or solar reflective pigments).

There is a continuing need to prevent the appearance of undesirablealgae growth on roofing shingles and other roofing materials in anefficient and cost-effective manner.

SUMMARY OF THE INVENTION

The present invention provides, an article, in particular a roofinggranule, which is photocatalytic in and of itself without a coating.

In one presently preferred embodiment, the present invention providesphotocatalytic roofing granules comprising a binder, inert mineralparticles, and photocatalytic particles. In this embodiment, the inertmineral particles and the photocatalytic particles are dispersed in thebinder. Preferably, the photocatalytic particles are selected from thegroup consisting of anatase titanium dioxide and zinc oxide. Further, itis preferred that the photocatalytic particles have an average particlesize from about 5 nanometers to 5 microns, and more preferably fromabout 5 nanometers to 100 nanometers. Preferably, the inert mineralparticles have an average particle size from about 0.1 micrometers to 40micrometers and more preferably from about 0.25 micrometers to 20micrometers. Preferably, the photocatalytic roofing granules have anaverage particle size from about 0.1 mm to 3 mm, and more preferablyfrom about 0.5 mm to 1.5 mm. Preferably, the binder is selected from thegroup consisting of silicate, silica, phosphate, titanate, zirconate,and aluminate binders, and mixtures thereof. Preferably, the binderfurther comprises an inorganic material selected from the groupconsisting of aluminosilicate and kaolin clay.

In another presently preferred embodiment, the present inventionprovides photocatalytic roofing granules comprising a porous bodycomprising inert mineral particles, and photocatalytic particles withinthe pores of the body. In this embodiment, the photocatalytic particlesare preferably selected from the group consisting of anatase titaniumdioxide and zinc oxide. Preferably, the photocatalytic particles have anaverage particle size from about 5 nanometers to 5 microns, and morepreferably from about 5 nanometers to 100 nanometers. Preferably, theinert mineral particles have an average particle size from about 0.1micrometers to 40 micrometers and more preferably from about 0.25micrometers to 20 micrometers. Preferably, the photocatalytic roofinggranules have an average particle size from about 0.1 mm to 3 mm, andmore preferably from about 0.5 mm to 1.5 mm. In one aspect, the porousbody comprises a plurality of mineral particles and a binder, and thebinder is preferably selected from the group consisting of silicate,silica, phosphate, titanate, zirconate, and aluminate binders, andmixtures thereof. Preferably, the binder in this aspect furthercomprises an inorganic material selected from the group consisting ofaluminosilicate and kaolin clay.

In another aspect, the present invention provides a process forpreparing photocatalytic roofing granules. In a first presentlypreferred embodiment, the process comprises providing a binder, inertmineral particles, and photocatalytic particles, dispersing the inertmineral particles and the photocatalytic particles in the binder to forma mixture, forming the mixture into granules, and curing the binder. Inanother presently preferred embodiment, the process comprises providinga binder and inert mineral particles to form a mixture; photocatalyticparticles, forming the mixture into porous granules, curing the binderto form a porous granule body, and dispersing the photocatalyticparticles in the pores of the granule body.

In another aspect, the present invention provides a process forpreparing photocatalytic roofing granules in which the process comprisesproviding ceramic particles; forming the ceramic particles into uncuredgranule bodies having an exterior surface; sintering the uncured granulebodies to form sintered granule bodies; and adhering photocatalyticparticles to the exterior surface of the sintered granule bodies to formphotocatalytic roofing granules. In one embodiment of this aspect of thepresent invention, the process further includes providing a sinteringbinder and mixing the sintering binder with the ceramic particles toform a mixture and subsequently forming the mixture including theceramic particles into uncured granule bodies. In one embodiment, thephotocatalytic particles are mechanically adhered to the exteriorsurface of the uncured granule bodies. In an alternative embodiment, theprocess further comprises mixing the photocatalytic particles with anexterior binder to form an exterior coating composition; applying theexterior coating composition to the cured granule bodies; and curing theexterior coating composition.

In another aspect, the present invention provides photocatalytic roofinggranules having an exterior surface, the roofing granules comprisingsintered ceramic particles; and photocatalytic particles; wherein atleast some of the photocatalytic particles are proximate the exteriorsurface of the roofing granules. Preferably, the photocatalyticparticles are selected from the group consisting of anatase titaniumdioxide and zinc oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional elevational representation of a roofinggranule according to a first embodiment of the present invention.

FIG. 2 is a schematic sectional elevational representation of a roofinggranule according to a second embodiment of the present invention.

FIG. 2 a is a partial fragmentary schematic sectional elevationalrepresentation of the roofing granule of FIG. 2.

FIG. 3 is a schematic sectional elevational representation of a roofinggranule according to a third embodiment of the present invention.

FIG. 4 is a schematic sectional elevational representation of a roofinggranule according to a fourth embodiment of the present invention.

FIG. 5 is a schematic sectional elevational representation of a roofinggranule according to a fifth embodiment of the present invention.

FIG. 5 a is a partial fragmentary schematic sectional elevationalrepresentation of the roofing granule of FIG. 6.

FIG. 6 is a partial fragmentary schematic sectional elevationalrepresentation of a roofing granule according to a sixth embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention provides roofing granules which include acomposite inorganic granule body and a plurality of photocatalyticparticles, such as anatase form of titanium dioxide or zinc oxide, withparticle sizes in nano-scale (5 to 100 nm) or larger (up to twomicrons).

In another embodiment, the present invention provides a compositegranule body that includes porosity. The porosity of the granule bodycan be of different sizes, shapes and forms (interconnected or isolatedporosity). In this embodiment, a plurality of photocatalytic particlesis immobilized within the pore structures of the granule body. Thephotocatalytic particles can be introduced into the pores by blending ofthe photocatalytic particles with other ingredients during the granulebody formation processes. Alternatively, a suspension of photocatalyticparticles or a solution of photocatalytic titania sol can be mixed withthe porous granule body, and the photocatalytic particles are drawn intothe pores by capillary action.

The mineral particles employed in the process of the present inventionare preferably chemically inert materials. The mineral particlespreferably have an average particle size of from about 0.1 micrometersto about 40 micrometers and more preferably from about 0.25 micrometersto about 20 micrometers. Stone dust can be employed as the source of themineral particles in the process of the present invention. Stone dust isa natural aggregate produced as a by-product of quarrying, stonecrushing, machining operations, and similar operations. In particular,dust from talc, slag, limestone, granite, marble, syenite, diabase,greystone, quartz, slate, trap rock, basalt, greenstone, andesite,porphyry, rhyolite, greystone, and marine shells can be used, as well asmanufactured or recycled manufactured materials such as ceramic grog,proppants, crushed bricks, concrete, porcelain, fire clay, and the like.Ceramic materials, such as silicon carbide and aluminum oxide ofsuitable dimensions can also be used.

“Green” or uncured photocatalytic roofing granules can be formed from amixture of mineral particles, photocatalytic particles and binder,ranging from about 95% by weight binder to less than about 10% by weightbinder, and the uncured photocatalytic roofing granules preferably areformed from a mixture that includes from about 10% to 40% by weightbinder.

The binder can be a binder selected from the group consisting ofsilicate, silica, phosphate, titanate, zirconate, and aluminate binders,and mixtures thereof. The binder can further comprise an inorganicmaterial selected from the group consisting of aluminosilicate andkaolin clay. In one aspect of the present invention, the binder is asoluble alkali metal silicate, such as aqueous sodium silicate oraqueous potassium silicate. The soluble alkali metal silicate issubsequently insolubilized by heat or by chemical reaction, such as byreaction between an acidic material and the alkaline silicate, resultingin cured photocatalytic roofing granules. The binder may also includeadditives for long term outdoor durability and functionality.

When an alkali metal-silicate binder such as sodium silicate is employedin the preparation of photocatalytic roofing granules, the binder caninclude a heat-reactive aluminosilicate material, such as clay, forexample, kaolin clay. Alternatively, it is possible to insolubilize thealkali metal silicate binder chemically by reaction with an acidicmaterial, for example, ammonium chloride, aluminum chloride,hydrochloric acid, calcium chloride, aluminum sulfate, and magnesiumchloride, such as disclosed in U.S. Pat. Nos. 2,591,149, 2,614,051,2,898,232 and 2,981,636, or other acidic material such as aluminumfluoride. The binder can also be a controlled release sparingly watersoluble glass such as a phosphorous pentoxide glass modified withcalcium fluoride, such as disclosed in U.S. Pat. No. 6,143,318. The mostcommonly used binder for conventional granule coating is a mixture of analkali metal silicate and an alumino-silicate clay material.

The mixture of mineral particles, photocatalytic particles and bindercan be formed into uncured photocatalytic roofing granules, using aforming process such as press, molding, cast molding, injection molding,extrusion, spray granulation, gel casting, pelletizing, compaction, oragglomeration. Preferably, the resulting uncured photocatalytic roofinggranules have sizes between about 50 micrometers and 5 mm, morepreferably between about 0.1 mm and 3 mm, and still more preferablybetween about 0.5 mm and 1.5 mm. The uncured photocatalytic roofinggranules can be formed using a conventional extrusion apparatus. Forexample, aqueous sodium silicate, kaolin clay, mineral particles, andphotocatalytic particles and water (to adjust mixability) can be chargedto a hopper and mixed by a suitable impeller before being fed to anextrusion screw provided in the barrel of the extrusion apparatus.Alternatively, the ingredients can b e charged to the extrudercontinuously by gravimetric feeds. The screw forces the mixture througha plurality of apertures having a predetermined dimension suitable forsizing roofing granules. As the mixture is extruded, the extrudate ischopped by suitable rotating knives into a plurality of uncuredphotocatalytic roofing granules, which are subsequently fired at anelevated temperature to sinter or densify the binder.

When the formed granules are fired at an elevated temperature, such asat least 250 degrees C., and preferably at 400 to 800 degrees C., theclay binder densifies to form strong particles.

Examples of clays that can be employed in the process of the presentinvention include kaolin, other aluminosilicate clays, Dover clay,bentonite clay, etc.

Examples of photocatalytic particles that can be employed in forming thephotocatalytic roofing granules of the present invention includephotocatalytic titanium oxide such as anatase titanium dioxide,photocatalytic copper oxide, photocatalytic vanadium oxide, andphotocatalytic zinc oxide. Preferably, the photocatalytic particlescomprise at least one photocatalytic particulate, preferably a metaloxide, comprising from about 0.1 to 20% by weight of the photocatalyticroofing granules. Moreover, it is preferred that the photocatalyticparticles have an average particle size of from 1 nm to 60 nm asdetermined by light scattering. Preferably, the at least onephotocatalytic particulate is anatase titanium dioxide.

Preferably, the photocatalytic particles are selected to have highphotoefficiency. In particular, the grain size and crystal phase of theparticles are preferably selected to enhance photoactivity. Further, thephotocatalytic particles or particulate preferably include selecteddopants to enhance photoactivity. For example, when the photocatalyticparticulate is nanocrystalline titanium dioxide, the particulate can beprepared as the anatase crystal phase, the particulate can be preparedas a mesoporous material, Fe(III), Nb(V), V(V) Pt and like dopants maybe included, noble metal nanodomains may be included, the surface of thetitanium dioxide can be treated to enhance diffusion of oxidizingspecies from the surface, and the like.

In yet another aspect of the present invention, the binder comprises achemically bonded cement, preferably, a chemically bonded phosphatecement. It is preferred in this aspect that the binder comprise achemically bonded phosphate cement prepared from a cementitious exteriorcoating composition including at least one metal oxide or a metalhydroxide slightly soluble in an acidic aqueous solution to providemetal cations and a source of phosphate anions. Preferably, the relativequantities of the at least one metal oxide or metal hydroxide and atleast one source of phosphate anion are selected to provide a curedcoating having a neutral pH, the coating composition being cured by theacid-base reaction of the at least one metal oxide or hydroxide and thesource of phosphate anions. Preferably, in this aspect the bindercomprises at least one metal oxide or metal hydroxide as a source ofmetal cations and at least one phosphate. Preferably, at least one metaloxide or metal hydroxide comprises at least one clay. Preferably, thebinder further includes colloidal silica.

Preferably, the at least one metal oxide or metal hydroxide is selectedfrom the group consisting of alkali metal oxides, alkaline earth metalhydroxides, aluminum oxide, oxides of first row transition metals,hydroxides of first row transition metals, oxides of second rowtransition metals, and hydroxides of second row transition metals. Morepreferably, the at least one metal oxide or metal hydroxide is selectedfrom the group consisting of magnesium oxide, calcium oxide, iron oxide,copper oxide, zinc oxide, aluminum oxide, cobalt oxide, zirconium oxideand molybdenum oxide. Preferably, the at least one metal oxide or metalhydroxide is sparingly soluble in an acidic aqueous solution. Inaddition, it is preferred that the at least one metal oxide or metalhydroxide comprise from about 10 to 30% by weight of the binder.

Preferably, the at least one phosphate is selected from the groupconsisting of phosphoric acid and acid phosphate salts. More preferably,the at least phosphate is selected from the group consisting ofphosphoric acid, and acid salts of phosphorous oxo anions, andespecially salts including at least one cation selected from the groupconsisting of ammonium, calcium, sodium, potassium, and aluminumcations. In particular, it is preferred that the at least one phosphatebe selected from the group consisting of phosphoric acid, ammoniumhydrogen phosphate, ammonium dihydrogen phosphate, potassium hydrogenphosphate, potassium dihydrogen phosphate, potassium phosphate, calciumhydrogen phosphate, calcium dihydrogen phosphate, magnesium hydrogenphosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate,aluminum hydrogen phosphate, aluminum dihydrogen phosphate, and mixturesthereof. Commercial grades of calcium phosphate salts, such “NSF”(normal super phosphate) and “TSP” (triple super phosphate) can also beused. Potassium dihydrogen phosphate (“monopotassium phosphate”),aluminum hydrophosphate (AlH₃(PO₄)·2H₂O), monoaluminum phosphate(Al(H₂PO₄)₃) and magnesium dihydrogen phosphate are especiallypreferred. Preferably, the at least one phosphate comprises from about10 to 60% by weight of the binder.

In this aspect of photocatalytic roofing granules according to thepresent invention, the cure of the binder depends on the composition ofthe chemically bonded cement. A broad range of cure conditions, rangingfrom rapid room temperature curing to low energy cures at moderatelyelevated temperatures to high energy cures at more elevated temperaturescan be attained by varying the metal oxide or hydroxide and thephosphate. Optionally, the reactivity of the metal oxide or hydroxidecan be reduced by calcining the metal oxide or metal hydroxide prior topreparing the binder. In addition, the pot life of the binder can beextended by the optional addition of a retardant such as boric acid.

Thus, in one aspect the present invention provides a process forpreparing photocatalytic roofing granules. In this aspect, the processcomprises providing a binder, inert mineral particles, andphotocatalytic particles; dispersing the inert mineral particles and thephotocatalytic particles in the binder to form a mixture; forming themixture into granules; and curing the binder.

In one presently preferred embodiment of the present invention, porousbase particles are provided. Particle synthesis allows properties of thephotocatalytic roofing granules to be tailored, such as the porosity ofthe granule and the distribution of the photocatalytic particles. Thebase particles are preferably prepared by mixing mineral particles witha suitable binder, such as a binder comprising an aluminosilicatematerial, such as clay (which is also, formally, composed of “mineralparticles,” but not as that term is used herein).

In another aspect of the present invention, photocatalytic roofinggranules are produced by an accretion process such as disclosed in U.S.Pat. No. 7,067,445, incorporated herein in its entirety by reference.The starting materials employed are mineral particles and binder, andoptionally photocatalytic particles. The starting materials arepreferably ground, if necessary, by ball milling or another attritionprocess, to form particles having a mean particle size of about 20microns or less, more preferably, about 15 microns or less, and mostpreferably about 10 microns or less, expressed in terms of particlediameter (or average diameter for non-spherical particles). The groundstarting materials are combined with a liquid, such as water, and mixedin an intensive mixer, such as an Eirich mixer (Eirich Machines Inc.,Gurnee, Ill. 60031) having a rotatable confinement vessel having arotatable table, or pan, and a rotatable impacting impeller. In anintensive mixer the rotatable table and impeller rotate in oppositedirections. Sufficient water or other liquid is added to causeessentially spherical pellets of the starting material mixture to beformed (about 15 to 40 weight percent water based on the startingmaterials). After such pellets have formed, a second mixture is added,and the mixture is further operated to cause accretion of the addedmaterial to the pellets being formed. The second mixture includesphotocatalytic particles and binder, and optionally mineral particlesand colorant material particles. The second mixture preferable comprisesup to 25 percent, and more preferably, from about 5 to 15 percent byweight, of the starting materials. The pellet so formed are then driedto a moisture content of less than about 10 weight percent, for example,in a drier at a temperature between about 100 degree C. and 300 degreesC. to form “green” roofing granules. The “green” roofing granules soformed are subsequently cured. Depending on the nature of the binder,the “green” granules can be cured by heating at an elevated temperatureto cure the binder. For example, when the binder comprises aqueoussodium silicate and kaolin clay, the “green” granules can be cured byheating at a temperature between about 400 degrees C. and 800 degrees C.to solidify the binder.

In another aspect of the present invention, photocatalytic roofinggranules are produced by an accretion process similar to that disclosedin U.S. Pat. No. 7,067,445. In this aspect of the present invention, thestarting materials employed are ceramic particles and a sinter binder,and optionally photocatalytic particles.

Suitable ceramic particles include oxides, such as aluminum oxides, suchas alumina, silicon oxides, such as silica, and mixtures thereof.Preferably, the ceramic particles comprise silica and alumina, andcomprise at least 80 percent by weight of the starting materials,expressed in terms of the calcined (essentially anhydrous) weight, andmore preferably, at least about 90 percent of the calcined weight.

“Calcined” as used herein refers to a heating process to which amaterial has been subjected to release water and other volatiles fromthe material, such as organic materials and chemically bound water suchwater of hydration. Ore materials that have been fully calcined exhibitvery low loss on ignition (“LOI”) and moisture content, for example,about 1 to 2 percent by weight or less. Uncalcined ore materials such asbauxites and clays can contain from about 10 to about 40 percentvolatiles. “Partially calcined” material typically exhibit totalvolatiles (LOI and moisture content) of about 5 to 8 percent. Typicalcalcination temperatures are usually less than 1000 degrees C.

The ceramic particles can be clays (predominantly hydrated alumina) suchas kaolin, diaspore clay, burley clay, flint clay, bauxitic clays,nature or synthetic bauxites, mixtures thereof and the like. The ceramicparticles can be calcined or partially calcined. The ceramic particlesare preferably formed from oxides, aluminates, and silicates, andpreferably comprise up to 50 percent by weight, more preferably at least90 percent by weight, and most preferably at least 90 percent by weightof the starting materials.

The starting materials can also include various sintering aids, such asbentonite clay, iron oxide, boron, boron carbide, aluminum diboride,boron nitride, boron phosphide, other boron compounds, or fluxes such assodium carbonate, lithium carbonate, titania, calcium carbonate, andsodium silicate, which materials can be added in amounts up to about 10percent by weight to aid in sintering.

In addition, a sintering binder, such as wax, a starch, or resin, suchas gelatinized cornstarch, polyvinyl alcohol, or mixture thereof, can beadded to the initial mixture to aid in pelletizing the mixture andincrease the green strength of the pellets prior to sintering. Thesintering binder can be added in an amount of about 0 to 6 percent byweight of the starting materials.

The starting materials are preferably ground, if necessary, by ballmilling or another attrition process, to form particles having a meanparticle size of about 20 microns or less, more preferably, about 15microns or less, and most preferably about 10 microns or less, expressedin terms of particle diameter (or average diameter for non-sphericalparticles). The ground starting materials are combined with a liquid,such as water, and mixed in an intensive mixer. Sufficient water orother liquid is added to cause essentially spherical pellets of thestarting material mixture to be formed (about 15 to 40 weight percentwater based on the starting materials). After such pellets have formed,a second mixture is added, and the mixture is further operated to causeaccretion of the added material to the pellets being formed. The secondmixture includes photocatalytic particles and sintering binder, andoptionally ceramic particles, sintering aid, and colorant materialparticles. The second mixture preferable comprises up to 25 percent, andmore preferably, from about 5 to 15 percent by weight, of the startingmaterials. The pellet so formed are then dried to a moisture content ofless than about 10 weight percent, for example, in a drier at atemperature between about 100 degree C. and 300 degrees C. to form“green” roofing granules.

The “green” roofing granules so formed are subsequently sintered in afurnace at a sintering temperature until a specific gravity of fromabout 2.1 to 4.1 grams per cubic centimeter is obtained, depending onthe composition of the starting materials, and the desired specificgravity of the roofing granules. Sintering generally causes a reductionof up to about 20 percent in pellet size as well as an increase inspecific gravity. Suitable sintering temperatures are generally about1150 degrees C. and above, more preferably at about 1300 degrees C.,still more preferably about 1500 degrees C., although sinteringtemperatures can be as high as 1600 degrees C.

Preferably, the curing or sintering temperature is selected so as toavoid loss or reduction of the photocatalytic activity of thephotocatalytic particles. For example, when the photocatalytic particlescomprise anatase titanium dioxide, it is preferred to employ a sinteringor curing temperature less than about 900 degrees C. to avoid a phasechange to the rutile crystal structure.

In another aspect of the present invention, porous base particles areformed, and photocatalytic particles are subsequently introduced intothe pores of the porous base particles. The porous base particles can beformed from a mixture of a binder and mineral particles, such asdescribed above, and at least one void-forming material. The at leastone void-forming material can be an organic material or inorganiccompound. Preferably, the void-forming material is selected so that itreleases gaseous material, such as by decomposing into gaseous products,at suitably elevated temperatures. The void-forming material preferablyreleases gaseous material at a temperature that is greater than 90degrees C. The void-forming material may, for example, release boundwater, or water of hydration, at the elevated temperature. In thealternative, the void-forming material may itself decompose at anelevated temperature, preferably at a temperature above about 150degrees C. Examples of void-forming materials include sugar, sugar-basedproducts such as candy “sprinkles,” crushed nuts (such as walnutshells), crushed corn and grains, carbon or graphite balls, syntheticand natural polymers, organic fibers, flame-retardants, organicperoxides and hydrated compounds. The void-forming material can beeither water-soluble or water-insoluble. Preferably, the void-formingmaterial comprises at least 0.1 percent by weight of the base particlesemployed to prepare the photocatalytic roofing granules. Preferably, thevoid-forming material has an average particle size no larger than about2 mm. The void-forming material preferably has an average particle sizefrom about 100 micrometer to about 400 micrometer. Mixtures ofvoid-forming materials can also be used, as well as mixture ofwater-soluble and water-insoluble void-forming material. The proportionsof mixtures of void-forming materials can be tailored to achieve desiredporosity characteristics for the resulting base particles. Thevoid-forming material preferably comprises a substance selected from thegroup consisting of ground walnut shells, sugar, and carbon black. Inone presently preferred embodiment of the present invention, thevoid-forming material comprises about 1.4 percent by weight of the baseparticles.

In this aspect of the present invention, the base particles are formedfrom the mineral particles, the at least one void-forming material, andthe binder, and the binder is cured, such as by firing at an elevatedtemperature, to provide inert, porous base particles. The porous baseparticles can then be treated with a suspension or slurry ofphotocatalytic particles in a suitable medium, which is drawn into theporous base particles by capillary action. The suspension medium issubsequently removed, as by drying, to form photocatalytic roofinggranules.

In yet another aspect of the present invention, an inert core materialis covered with a coating composition of a mixture of binder, mineralparticles, and at least one void-forming material, and the coatingcomposition is cured to provide base particles having a solid inertmineral core and a porous exterior coating. Photocatalytic particles aresubsequently introduced into the pores of the exterior coating.

The inert mineral core material can be a suitably sized mineral particlesuch as described above, or in the alternative, the mineral corematerial can be a solid or hollow glass spheres. Solid and hollow glassspheres are available, for example, from Potters Industries Inc., P. 0.Box 840, Valley Forge, Pa. 19482-0840, such as SPHERIGLASS® solid “A”glass spheres product grade 1922 having a mean size of 0.203 mm, productcode 602578 having a mean size of 0.59 mm, BALLOTTINI impact beadsproduct grade A with a size range of 600 to 850 micrometers (U.S. seivesize 20-30), and QCEL hollow spheres, product code 300 with a meanparticle size of 0.090 mm. Glass spheres can be coated or treated with asuitable coupling agent if desired for better adhesion to the binder ofthe coating composition.

Referring now to the drawings, in which like reference numerals refer tolike elements in each of the several views, there are shownschematically in FIGS. 1, 2, 3, 4 and 5 examples of photocatalyticroofing granules according to the present invention. FIG. 1 is aschematic cross-sectional representation of a first embodiment ofphotocatalytic roofing granule 10 according to the present invention.The photocatalytic roofing granule 10 comprises a plurality of inertmineral particles 12 and photocatalytic particles 14 dispersed in abinder 16. The inert mineral particles 12 and binder 16 togethercomprise an composite inorganic granule body. The photocatalytic roofinggranule 10 has an exterior surface 18. Photocatalytic activity isprovided to the photocatalytic roofing granule 10 by virtue of thephotocatalytic particles 14 provided at or proximate the exteriorsurface 18 of the photocatalytic roofing granule 10. The photocatalyticroofing granule 10 can be formed by extrusion, agglomeration, rollcompaction or other forming techniques. While the photocatalytic roofinggranule 10 is shown schematically as a sphere in FIG. 1, photocatalyticroofing granules according to the present invention can assume anyregular or irregular shape. After formation, depending on binderchemistry, the photocatalytic roofing granule 10 can be fired at 250degrees C. or higher, preferably from 400 degrees C. to 800 degrees C.,to insolubilize the binder 16. The particle size of the photocatalyticroofing granule 10 preferably ranges from about 0.1 mm to 3 mm, and morepreferably from about 0.5 mm to 1.5 mm. The inert mineral particles 12are minute particulates or dust, such as for example, particulates ofrhyolite, syenite and other rock sources formed as a byproduct fromquarry, crushing and similar operations. The inert mineral particles 12preferably have a particle size ranging from about 0.1 micrometer to 40micrometers, and more preferably from about 0.25 micrometer to 20micrometers. The binder 16 is preferably selected from the groupconsisting of silicate, silica, phosphate, titanate, zirconate andaluminate binders, and mixtures thereof. The binder content of thephotocatalytic roofing granule 10 preferably ranges from 10% to 90% byweight. In addition, aluminosilicate, kaolin clay and other inorganicmaterials can be added to the binder 16 to improve the mechanical,chemical, or physical properties of the photocatalytic roofing granule10.

FIG. 2 is a schematic cross-sectional representation of a secondembodiment of photocatalytic roofing granule 20 according to the presentinvention. The photocatalytic roofing granule 20 comprises a pluralityof inert mineral particles 22 and photocatalytic particles 24 dispersedin a binder 26, and has an inner surface 28, and an exterior coatinglayer 30 formed on the inner surface 28. The exterior coating layer 30is substantially transparent to ultraviolet radiation, such as, forexample, at least 80 percent transparent to ultraviolet radiation. Theexterior coating layer 30 can be formed, for example, from a curablecoating composition such as disclosed in International PatentPublication WO/2003/085058 comprising an organohydrogenpolysiloxane, analkenyl functional polysiloxane, and an ultraviolet radiation absorbingphotocatalyst, or such as disclosed in U.S. Pat. No. 6,204,304,incorporated herein by reference, and providing an exterior coatinglayer 30 having a high level of transparency to ultraviolet radiation,such as a coating that allows from 70% to 99% of radiation ofwavelengths from 240 nm to 275 nm to pass through. Preferably, theexterior coating layer 30 is thin enough to permit photocatalyticparticles 24 proximate the inner surface 28 to provide photocatalyticaction at the surface of the photocatalytic roofing granules 20. Forexample, the exterior coating layer 30 has a thickness of from about 20micrometers to 200 micrometers. The exterior coating layer 30 can alsoinclude particulate colorants 29 or dyes to provide desired aestheticeffects, better seen in the partial fragmentary schematiccross-sectional view of FIG. 2 a.

FIG. 3 is a schematic cross-sectional representation of a thirdembodiment of a photocatalytic roofing granule 31 according to thepresent invention. The photocatalytic roofing granule 31 comprises aninert composite mineral body or granule body 32 having a plurality ofpores 34 formed therein, and a plurality of photocatalytic particles 36dispersed in the pores 34, and an exterior surface 38. Photocatalyticactivity is provided to the photocatalytic roofing granule 31 by virtueof the photocatalytic particles 36 provided at or proximate the exteriorsurface 38 of the photocatalytic roofing granule 31. FIG. 4 is aschematic cross-sectional representation of a fourth embodiment of aphotocatalytic roofing granule 40 according to the present invention.The photocatalytic roofing granule 40 comprises an inert mineral baseparticle 42 having an outer surface 44 coated with an inert minerallayer 52 having a plurality of pores 54 formed in the inert minerallayer 52, and a plurality of photocatalytic particles 56 dispersed inthe pores 54.

Photocatalytic activity is provided to the photocatalytic roofinggranule 40 by virtue of the photocatalytic particles 56 provided at orproximate the exterior surface 58 of the photocatalytic roofing granule40.

FIG. 5 is a schematic cross-sectional representation of a fifthembodiment of photocatalytic roofing granule 60 according to the presentinvention. The photocatalytic roofing granule 60 comprises a pluralityof inert mineral particles 62 and dispersed in a binder 66 as well as anexterior layer 70 of photocatalytic particles 64 dispersed in binder 66proximate the surface of the roofing granule 60, and formed by aparticle accretion process in an intensive mixer. The exterior layer 70can have a thickness of from about 20 micrometers to 200 micrometers.The exterior layer 70 can also include particulate colorants 69 or dyes,better seen in the partial fragmentary view of FIG. 5 a. In anotheraspect of this embodiment, the inert mineral particles 62 can compriseceramic particles which are mixed with a sintering binder, formed intogreen cores by an agglomeration process (not shown). The green cores aresubsequently sintered together at elevated temperature to form sinteredcores, to which cores are subsequently adhered photocatalytic particles,such as by mixing photocatalytic particles with an exterior binder toform an exterior coating composition which is subsequently applied tothe exterior of the sintered cores and cured (not shown).

FIG. 6 is a fragmentary schematic cross-sectional representation of asixth embodiment of photocatalytic roofing granule 80 according to thepresent invention. The photocatalytic roofing granule 80 comprises aplurality of sintered ceramic particles 82 as an exterior layer 90 ofphotocatalytic particles 84 sintered to the ceramic particles 82proximate to the surface the roofing granule 80, and formed by aparticle accretion process in an intensive mixer to form green pellets,followed by sintering at an elevated temperature. The exterior layer 90can have a thickness of from about 20 micrometers to 200 micrometers.The exterior layer 90 can also include particulate colorants 89,sintered to the ceramic particles 82 and/or photocatalytic particles 84.

The photocatalytic roofing granules of the present invention can beemployed in the manufacture of roofing products, such as asphaltshingles, using conventional roofing production processes. Typically,bituminous roofing products are sheet goods that include a non-wovenbase or scrim formed of a fibrous material, such as a glass fiber scrim.The base is coated with one or more layers of a bituminous material suchas asphalt to provide water and weather resistance to the roofingproduct. One side of the roofing product is typically coated withmineral granules to provide durability, reflect heat and solarradiation, and to protect the bituminous binder from environmentaldegradation. The photocatalytic roofing granules of the presentinvention can be mixed with conventional roofing granules, and thegranule mixture can be embedded in the surface of such bituminousroofing products using conventional methods. Alternatively, thephotocatalytic roofing granules of the present invention can besubstituted for conventional roofing granules in manufacture ofbituminous roofing products.

Bituminous roofing products are typically manufactured in continuousprocesses in which a continuous substrate sheet of a fibrous materialsuch as a continuous felt sheet or glass fiber mat is immersed in a bathof hot, fluid bituminous coating material so that the bituminousmaterial saturates the substrate sheet and coats at least one side ofthe substrate. The reverse side of the substrate sheet can be coatedwith an anti-stick material such as a suitable mineral powder or a finesand. Roofing granules are then distributed over selected portions ofthe top of the sheet, and the bituminous material serves as an adhesiveto bind the roofing granules to the sheet when the bituminous materialhas cooled. The sheet can then be cut into conventional shingle sizesand shapes (such as one foot by three feet rectangles), slots can be cutin the shingles to provide a plurality of “tabs” for ease ofinstallation and aesthetic effect, additional bituminous adhesive can beapplied in strategic locations and covered with release paper to providefor securing successive courses of shingles during roof installation,and the finished shingles can be packaged. More complex methods ofshingle construction can also be employed, such as building up multiplelayers of sheet in selected portions of the shingle to provide anenhanced visual appearance, or to simulate other types of roofingproducts. Alternatively, the sheet can be formed into membranes or rollgoods for commercial or industrial roofing applications.

The bituminous material used in manufacturing roofing products accordingto the present invention is derived from a petroleum-processingby-product such as pitch, “straight-run” bitumen, or “blown” bitumen.The bituminous material can be modified with extender materials such asoils, petroleum extracts, and/or petroleum residues. The bituminousmaterial can include various modifying ingredients such as polymericmaterials, such as SBS (styrene-butadiene-styrene) block copolymers,resins, flame-retardant materials, oils, stabilizing materials,anti-static compounds, and the like. Preferably, the total amount byweight of such modifying ingredients is not more than about 15 percentof the total weight of the bituminous material. The bituminous materialcan also include amorphous polyolefins, up to about 25 percent byweight. Examples of suitable amorphous polyolefins include atacticpolypropylene, ethylene-propylene rubber, etc. Preferably, the amorphouspolyolefins employed have a softening point of from about 130 degrees C.to about 160 degrees C. The bituminous composition can also include asuitable filler, such as calcium carbonate, talc, carbon black, stonedust, or fly ash, preferably in an amount from about 10 percent to 70percent by weight of the bituminous composite material.

Various modifications can be made in the details of the variousembodiments of the processes, compositions and articles of the presentinvention, all within the scope and spirit of the invention and definedby the appended claims.

1. Photocatalytic roofing granules comprising: (a) a binder; (b) inertmineral particles; and (c) photocatalytic particles selected from thegroup consisting of anatase titanium dioxide and zinc oxide; the inertmineral particles and the photocatalytic particles being dispersed inthe binder, the granules having an average particle size from about 0.1mm to 3 mm.
 2. (canceled)
 3. Photocatalytic roofing granules accordingto claim 1 wherein the photocatalytic particles have an average particlesize from about 5 nanometers to 5 microns.
 4. (canceled) 5.Photocatalytic roofing granules according to claim 1 wherein the inertmineral particles have an average particle size from about 0.1micrometers to 40 micrometers.
 6. (canceled)
 7. (canceled)
 8. (canceled)9. Photocatalytic roofing granules according to claim 1 wherein thebinder is selected from the group consisting of silicate, silica,phosphate, titanate, zirconate, and aluminate binders, and mixturesthereof.
 10. Photocatalytic roofing granules according to claim 9wherein the binder further comprises an inorganic material selected fromthe group consisting of aluminosilicate and kaolin clay. 11.Photocatalytic roofing granules comprising: (a) a porous body comprisinginert mineral particles; and (b) photocatalytic particles selected fromthe group consisting of anatase titanium dioxide and zinc oxide withinthe pores of the body, the granules having an average particle size fromabout 0.1 mm to 3 mm.
 12. (canceled)
 13. Photocatalytic roofing granulesaccording to claim 11 wherein the photocatalytic particles have anaverage particle size from about 5 nanometers to 5 microns. 14.(canceled)
 15. Photocatalytic roofing granules according to claim 11wherein the inert mineral particles have an average particle size fromabout 0.1 micrometers to 40 micrometers.
 16. (canceled)
 17. (canceled)18. (canceled)
 19. Photocatalytic roofing granules according to claim 11further comprising a binder selected from the group consisting ofsilicate, silica, phosphate, titanate, zirconate, and aluminate binders,and mixtures thereof.
 20. Photocatalytic roofing granules according toclaim 19 wherein the binder further comprises an inorganic materialselected from the group consisting of aluminosilicate and kaolin clay.21. (canceled)
 22. (canceled)
 23. A process for preparing photocatalyticroofing granules, the process comprising: (a) providing a binder andinert mineral particles to form a mixture; photocatalytic particles; (b)forming the mixture into porous granules; (c) curing the binder to forma porous granule body; and (d) dispersing the photocatalytic particlesin the pores of the granule body.
 24. A roofing product comprisingphotocatalytic roofing granules according to claim
 1. 25. A process forpreparing photocatalytic roofing granules, the process comprising: (a)providing ceramic particles; (b) forming the ceramic particles intouncured granule bodies having an exterior surface; (c) sintering theuncured granule bodies to form sintered granule bodies; and (d) adheringphotocatalytic particles to the exterior surface of the sintered granulebodies to form photocatalytic roofing granules.
 26. A process accordingto claim 25 further comprising providing a sintering binder and mixingthe sintering binder with the ceramic particles to form a mixture andsubsequently forming the mixture including the ceramic particles intouncured granule bodies.
 27. A process according to claim 25 wherein thephotocatalytic particles are mechanically adhered to the exteriorsurface of the uncured granule bodies.
 28. A process according to claim25 further comprising: (a) mixing the photocatalytic particles with anexterior binder to form an exterior coating composition; (b) applyingthe exterior coating composition to the cured granule bodies; and (c)curing the exterior coating composition.
 29. (canceled)
 30. (canceled)